The replacement of a missing bone fragment following resection of a tumour or major trauma often uses operating techniques designed to promote the growth or development of the adjacent healthy section of bone tissue.
One of these comprises exerting tension on the bone callus resulting from an osteotomy in order to lengthen it by an amount corresponding to the bone loss.
This technique, which has been considerably developed by the Russian practitioner Ilizarov, was initially used for the long bones or at least for bone fragments of generally cylindrical shape.
The external distractor described by this author in U.S. Pat. No. 4,978,348 published on 18 Dec. 1990 appears to offer sufficient flexibility of use to enable it to be used both for restoring the shape of the hand, by stretching the parts of phalanges remaining after amputation, and for the reconstruction of the mandible, for example.
The system comprises a set of threaded rods which may be assembled to fit the shape of the bone or set of bones being treated. The threaded rods carry pin-supporting carriages the adjustable spacings of which allow the bone distraction to take place. The pins of each carriage are tightened by a nut between washers threaded onto a hollow bolt engaging on one of the rods. The latter have a flat section cooperating with gudgeons of the hollow bolts for securing the pin supports against rotation relative to the rods.
The apparatus is simple but, as the carriages are held in place on the rods by locknuts, two spanners are needed to move them. Moreover, the locking and unlocking of a locknut requires a relatively complex simultaneous manipulation of these two spanners.
The external osteosynthesis fixator disclosed in Utility Certificate Application FR 2,671,479 in the name of the company Hit Medica, published on 17 Jul. 1992, is based on the principle of the turnbuckle. The two spindle supports are thus simply moved apart by manipulating the central part of the rod, the ends of which are threaded in opposite directions.
Based on another principle, the apparatus described in U.S. Pat. No. 4,848,368 published on 18 Jul. 1989 in the name of R. Kronner can according to the applicant be manoeuvred using a single knurled wheel. In this case the patient himself can perform the task.
The two systems mentioned above may be used for the distraction of straight bones, be they long or short, but they are not suitable for curved bones such as, for example, the bones of the skull, the mandibles or the ribs.
An apparatus which is specially adapted for the curved bone segments of jaws, face or cranium is described in German Patent Application DE 195 03 609 published on 10 Aug. 1995 in the name of Messrs Normed Medizin Technik Vertriebs. The apparatus is formed from a plurality of racks on which pin-supporting carriages are fixed. The racks are articulated to one another so that the assembly fits the shape of the bone on the outside. A carriage can be moved along a rack using a screw engaging therewith.
The joints between the racks comprise spherical connections, which has the advantage of allowing the assembly to be made in any shape. However, these joints have the serious disadvantage of restricting the movement of the carriages.
A threaded, curved, continuous rail which does not limit the movement of the pin-supporting carriages is disclosed in Japanese Patent Application JP 09-215699 published on 19 Aug. 1997 in the name of Messrs Nagoya Rashi Seisakusho.
According to the embodiment shown in the Figures accompanying the application, the rail has a square cross section and is threaded on each of its edges. In the example of use described, the rail is fixed to the right-hand side of a mandible and follows the contour thereof, from back to front, without going past the start of the chin.
However, it appears technically impossible for the rail described in JP application 09-215699 to follow the contour of the mandible beyond the start of the chin. The small radius of curvature at the chin would obviously result in a substantial deformation of the threads of the rod and prevent the rotation of the nuts which control the movement of the carriages.
In fact, in the position shown in the drawings which accompany JP application 09-215699, the threads inside the curve followed by the rod are necessarily compressed, while the outer threads are moved apart.
As a result, the curved rail would appear to be usable only for repairing a side part of the mandible, as shown in the main Figure, and certainly not for distraction of the median part of the mandible.
Consequently, it is apparent from the prior art as described in the documents cited above that external bone distractors adapted for the reconstruction of curved bone segments, notably the bones of the jaw, face and cranium, are known, but at present there is no osteogenic distraction device which has features that fully meet the needs of maxillo-facial surgery.